The largest land animal that ever lived is likely Amphicoelias fragillimus, a sauropod dinosaur known only from a single vertebra fossil fragment, measuring 1.5 m (5 ft) in height, which has since been lost. In its complete form, it was estimated the entire vertebrae fossil would have been 2.7 m (8.8 ft) tall. A gigantic fossilized femur was also discovered near the vertebra.
Extrapolating from the size of the femur, and assuming A. fragillimus had proportions similar to a typical diplodocid (a family of long and slender sauropods), its estimated length (including tail) was 58 m (190 feet), much longer than the blue whale, often cited as the largest animal that has ever lived, at only 30-33 m (98-110 ft) in length. Being slender, however, its weight would have been only about 130 tons, less than the 195 ton record for a blue whale or the 190-240 ton estimate for the weight of Bruhathkayosaurus (whose name means "huge-bodied lizard"), another sauropod.
Because of the unknown whereabouts of the A. fragillimus fossils and the tenuous method of extrapolation, its claims to the title of largest animal ever are often challenged. If the fossil really existed and the extrapolation is valid, then A. fragillimus is not only the largest land animal ever to have lived, but the largest animal in general. For decades, it was thought that Brachiosaurus, a sauropod with its largest individuals possessing a length of about 29 m (96 ft), was not only the largest land animal, but approached the maximum possible weight a land animal could be (30-60 tons) and still support itself without needing to be suspended in water. Yet modern estimates of sauropod weight cite seven dinosaurs with greater weights than Brachiosaurus, most clustered in the 60-100 ton range: Sauroposeidon, Antarctosaurus, Paralititan, Argentinosaurus, Puertasaurus, A. fragillimus, and Bruhathkayosaurus.
So what you consider the largest land animal to have ever lived depends on several factors: do you believe that the A. fragillimus fossil really existed and that the standard extrapolation of its size is correct? Or is weight the most important determinant of world’s largest animal? If so, your answer could be either A. fragillimus, with a length of 58 m (190 ft.) or Bruhathkayosaurus, with a top weight approaching 240 tons. For comparison, the largest blue whale on record had a length of 33.5 m (110 ft) and a weight of 195 tons.
Because both the vertebra and femur and nowhere to be found, the claims of A. fragillimus’s size are hazy and uncertain. The extrapolations used in projecting the size and weight of both A. fragillimus and Bruhathkayosaurus are mired in controversy, so they sometimes go unrecognized as the largest and most massive animals, being replaced by the contemporary species of which we have the most knowledge, the blue whale. Reaching a consensus as to which land animal was truly the largest will certainly require locating more fossils.